This invention relates to wheel balancers and in particular to improved drive systems, safety circuitry, and displays working in conjunction with said drive systems for wheel balancers.
The determination of unbalance in vehicle wheels is carried out by an analysis with reference to phase and amplitude of the mechanical vibrations caused by rotating unbalanced masses in the wheel. The mechanical vibrations are measured as motions, forces, or pressures by means of transducers, which convert the mechanical vibrations to electrical signals. Each signal is the combination of fundamental oscillations caused by imbalance and noise.
It is believed that the drive systems for currently available balancers could be improved to aid in operation. For example, prior art balancers typically require the operator to manually rotate the wheel/tire assembly to the desired position for weight placement and/or runout correction. These balancers then use a manual brake or the motor itself to temporarily hold the shaft in the desired position. However, such a system could be improved. Manual rotation to the desired position is less than satisfactory since it requires the operator to interpret the balancer display correctly. Moreover, manual rotation itself is not desirable, since it ties up the operator's time and attention. In conventional systems, however, the balancer motor cannot be used to rotate the wheel/tire assembly to the correct position since available motor controllers used in balancers are incapable of performing this function.
Using the motor itself to provide a braking action is not completely satisfactory either. Such braking is normally accomplished by applying rectified alternating current to an AC motor. This method is inherently subject to error. The actual stopping position may be incorrect if the tire is larger than average or turning too fast for the "brake" to respond. Moreover, although currently available motor braking systems stop the wheel in approximately the correct position, they do not actual hold the tire in position since the motor would heat up if the "brake" was left on. With conventional equipment, a wheel/tire assembly with sufficient static imbalance to overcome its own inertia, therefore, can roll away from the braked dynamic weight position as soon as the braking energy is released. In addition, electrical braking systems are usually of little use when power is removed from the circuit, as could occur should a power failure take place.
Currently available balancers could also be improved in another way. Presently, the balancer shaft position is sensed and the resulting signal is supplied to the control circuit. The control circuit typically analyzes the signal using software to determine if certain conditions (excessive rpm, excessive acceleration, etc.) exist. These systems are not foolproof, and could be improved.
Even when a wheel/tire assembly is balanced, non-uniformity in the construction of the tire as well as runout in the rim can cause significant vibration forces as the wheel rolls under vehicle load. Most tire manufacturers inspect their tires on tire uniformity machines and grind rubber off the tires as required to improve rolling characteristics of the tires. Even after this procedure, tires will often produce vibration forces (not related to imbalance) of 20 pounds as they roll on a smooth road. To put this in perspective of balancing, a 0.8 ounce balance weight is required to produce a 20 pound vibration force on a typical wheel traveling at 70 mph.
Prior art balancers are also not well equipped to take into account and correct for variations in uniformity of the wheel rim and the tire. It would be desirable, for example, to place a measured amount of imbalance in a wheel to counter tire non-uniformity forces or to detect and mark the position on a tire which should be matched to a corresponding position on the rim to reduce vibration due to non-uniformity of either or both. To the extent that presently available balancers do measure rim and tire runout, it is believed that the information they acquire is not particularly useful to the operator. In particular, presently available balancers which do measure runout generally display that runout to the user in the form of sine waves referenced to some arbitrary point. For a conventional system, which typically measures radial runout of both rims, this results in two (basically incomprehensible) sine waves. Such a system could be improved.